Circuit breakers are devices that serve to connect and disconnect current-carrying phase conductors in a three-phase network as needed. Whenever a sensor detects an error in the network, which typically manifests itself in the form of a voltage drop or a corresponding change in the current, such a circuit breaker opens one or more of the three phases, thus preventing the further flow of current. Such a circuit breaker can also be used as a reclosing switch in order to once again connect the current-carrying contacts.
Typically, such circuit breakers are configured as mechanical switches. The dimensioning is essentially determined by the maximum breaking capacity, which can amount to several thousand MVA. The switch uses the periodically occurring current zero crossings for switching off. Nevertheless, so-called electric arcs occur when the contacts are disconnected. In order to be able to dissipate these discharges in a controlled manner, the actual mechanical contacts are normally arranged in an arc quenching chamber, which is filled with an inert, electrically insulating gas such as, for example, SF6. Such a structure is described, for instance, in the ABB Review March 2002, pages 34 to 40.
Although mechanical circuit breakers for generators are capable of handling the power that is present in high-voltage networks, even with repeated closing and opening procedures of the circuit, they do have drawbacks, on the one hand, fundamentally because of their mechanical nature (wear) and, on the other hand, because of the fact that conventional circuit breakers require switching times (actuating signal until the voltage returns) of 60 to 120 ms. Accordingly, unavoidably high generator currents occur before the switch can effectuate the interruption.